The K mount fit on an EF adaptor but later it didn't fit in Metabones Speedbooster. I notice there was a tooth-shaped plastic on the back of the K mount, which made the lens unable to fit in Metabones Speedbooster. Therefore I had to saw out that piece of plastic, in order to make the lens work. But overall the lens is decent.
I've been looking for a good, cheap, 50mm fixed lens for a while now. Pentax has always been a cheap, yet high quality brand for a long time, and has kept the same K-mount type for a long time as well. Finding used lenses, like the f/1.4 from Asahi, is a fantastic thing for beginning photographers who want excellent quality but at affordable price, and even professionals who just want something to play around with and practice framing their shots. My only regret here is not buying one sooner. (And not being able to afford the f/1.2 ππ)